This invention relates to a method and an apparatus for performing the method of inducing compressive residual stress along the surface of a part and, more particularly, to a method and an apparatus for performing the method of burnishing or deep rolling a surface of a part whereby the magnitude and penetration of compressive residual stress achieved is greater than that achieved by conventional burnishing.
Surface residual stresses are known to have a major affect upon the fatigue and stress corrosion performance of components or parts in service. Tensile residual stresses that can be developed during manufacturing processes, such as grinding, turning, or welding are well known to reduce both fatigue life and increase sensitivity to corrosion-fatigue and stress corrosion cracking in a wide variety of materials. It is also known that compressive residual stresses induced in the surface of a part can increase its fatigue life and reduce its susceptibility to corrosion-fatigue and stress corrosion cracking. However, the benefit of inducing a layer of surface compression in reducing susceptibility to stress corrosion, cracking, fatigue, and corrosion-fatigue is lost if the layer of compression relaxes with time in service.
Many components and parts of practical interest are subject to high tensile cyclic loads or high mean loads that often lead to fatigue, corrosion fatigue, stress corrosion, or a combination of such failure modes. Therefore, it would be desirable to be able to introduce a layer of compressive residual stress along the surface of a part that will not relax significantly over time.
A method that has been developed and is widely used in industry to improve surface finish as well as fatigue life and corrosion resistance of a part by inducing a layer of compressive residual stress along its surface of the part is known as burnishing. During the burnishing process, the surface of a part is deformed by a rotary or sliding burnishing member that is pressed against the part in order to compress the microscopic peaks formed along the surface of the part into adjacent hollows. Burnishing thereby operates to develop compressive stresses by yielding the surface of the part in tension so that it returns to a state of compression following deformation. Burnishing tools comprising various wheel or roller burnishing member configurations have been developed for cold working a part and to induce a state of compressive stress and improved surface finish to the part. In addition, “deep rolling” and “low plasticity burnishing” processes have also been developed for producing deep layers of compressive stress that approach the yield strength of the material and which extend to over a millimeter into the surface. However, the deformation mechanism for producing such compressive stresses is based on hertzian loading and will generally produce maximum compression below the surface of the part. In many high strength or work hardening materials, the stresses produced along the upper surface by these burnishing methods can be far less than the subsurface maximum, often being close to or having zero compression at the upper surface. Processes have therefore been developed to increase the stress levels at the upper surface of a part. Such processes include removing a thin layer of material from the surface of the part, such as by etching, electropolishing, or some other non-tensile stress forming process; or a post treatment, such as shot peening, grit blasting, or similar compression producing treatments, to render the surface more highly compressive. Unfortunately, both approaches require a secondary surface treatment unrelated to the original burnishing process thereby adding time, cost, and the potential for damage and the loss of the part during manufacture.
With respect to shot peening operations, secondary peening operations have been used to improve surface compression. For example, to increase the state of surface compression in a part, secondary peening operations have been performed using small glass or ceramic shot following conventional steel shot peening with larger shot. Shot peening while being relatively inexpensive and preferred for many applications, is often unable to obtain the necessary coverage of the part without overlaping areas of impingement. Such overlapping often results in relatively large amount of cold working which may leave the surface compressive layer susceptible to stress relaxation. Further, shot peening is often unacceptable for use in manufacturing parts requiring a superior finish, localized or relatively complex compressive stress zones or patterns, or requiring a greater depth of compressive stress penetration.
Consequently, it would be desirable to have a relatively inexpensive and time efficient method and apparatus for implementing the method of improving the physical properties of a part by increasing the magnitude and penetration of compressive stress on the surface that would not significantly relax over time. It would also be desirable to have a method and an apparatus that would be effective for use with complex shaped surfaces and without detracting from the finish of the surface, and which could be performed relatively inexpensively and in a single pass.